U.S. Pat. No. 3,824,002 issued to Terry D. Beard, entitled "Alternating Current Liquid Crystal Light Valve" and assigned to the present assignee discloses the basic principles of operation of an alternating current liquid crystal light valve which requires that a photoconductor be impedance-matched to the liquid crystal, the photocapacitance of the photoconductor being modulated in response to an input light.
U.S. Pat. No. 3,976,361 issued to Fraas et al entitled "Charge Storage Diode with Graded Defect Density Photocapacitive Layer" and assigned to the present assignee discloses a different photoconductor for a similar light valve. The photoconductor consists of a charge storage semiconductor diode with a graded band gap layer increasing the optical absorption coefficient of the region near the rectifying junction to permit the storage of charge.
U.S. patent application Ser. No. 691,652 filed on June 1, 1976, by Jan Grinberg et al, entitled "Silicon Single Crystal Charge Storage Diode" and assigned to the present assignee describes a species of the generic invention of U.S. Pat. No. 3,976,361. This invention presently stands allowed and will issue in due course. It discloses a charge storage photodiode silicon substrate which is doped with a slow recombination center element such as silver, to combine the advantages of a highly developed silicon manufacturing technology with the high liquid crystal stability under ac operating conditions. The inventions disclosed and claimed in the aforementioned patents and patent application represent significant advances in this art as explained in detail in said patents and patent application. Our invention extends the developments in this area of technology and incorporates a concept that has applications extending considerably beyond the area of liquid crystal light valves.
For example, some of the present photoactive liquid crystal light valves made according to the teachings of U.S. Pat. No. 3,976,361 use a thin film of CdS which is driven with alternating current. The photodetector acts as a light-activated voltage gate. The thin film structure is designed to accept the major portion of the drive voltage when the photoconductor is unilluminated; the portion of the voltage that falls across the liquid crystal is below the threshold for activation of the liquid crystal electro-optic effect. When light falls on the photoconductor, its impedance drops, thereby switching the voltage from the photoconductor onto the liquid crystal and driving the liquid crystal into its activated state. Due to the high lateral impedance of the thin films, there is very little spread of the light and of its concomitant liquid crystal electro-optic effect. As a result, the light activation process is a high resolution process, so that the device can accept photographic quality images for transfer to an intense beam of light. Among the limitations of such a structure include relatively low switching ratios, undesirable memory effects and relatively slow response time. The switching ratio is defined as the ratio of the current through the light valve in the photoactivated state to the light valve current at the dark threshold. The fastest CdS photodetector is limited to a switching ratio of approximately 2:1, while a fast liquid crystal response time requires switching ratios in excess of 5:1. Furthermore, the sensitivity of the CdS photodetector is such that the primary image source has to be driven hard, which can cause resolution degradation, loss of contrast and an unnecessary system complexity.